The process of drying granulated thermoplastic polymer and copolymer materials can be divided into two stages. The first stage includes preliminary drying of materials containing 10 to 20 percent by weight of moisture which is mainly unbound. At this stage, the effectiveness of the drying process is directly proportional to the amount of thermal energy and the time during which the product is exposed to heat. The second stage is a stage of complete drying of materials containing less than one percent of moisture; this is mainly capillary and absorption, or bound, moisture, and the process at this stage is a markedly exponential process.
The first stage of the drying process requires high power levels of superhigh frequency oscillation, i.e. a considerable power input, but the time of exposure is very short, ranging from a few seconds to a few minutes, so there is no risk of the granules being destroyed or fused at the surface. The complete drying carried out during the second stage does not require a great power input, keeping in mind that the heat capacity of the material is almost independent of the moisture content, while the absorptivity is largely determined by the material itself. The heating temperature is determined by the type of changes in the tangent of the dielectric loss angle tg .delta. and the fusion temperature. During the second stage, the drying time amounts to scores of minutes.
Thus it may be advisable that the drying processes should be divided into two stages to be carried out in two separate superhigh frequency apparatus which may be connected by some conveyor means, for example, a pneumatic conveyor.
There is known a method for drying polymer materials, which is effected with the aid of a microwave applicator with a conveyor. According to this method, granules of a polymer material are introduced into a resonating chamber and moved through that chamber by an air flow which removes moisture and is let outside the resonating cavity. Vibration is produced to facilitate the motion of granules.
The apparatus for carrying out the above method is a microwave applicator provided with a conveyor for transportation of the product. The apparatus comprises a multimode resonance chamber accomodating a tray having holes therein. Arranged at the opposite ends of the chamber is an inlet and outlet intended for changing and discharging the product, respectively. The inlet and the outlet each include a set of lengths of pipe or waveguides with frequency ranges beyond those of superhigh frequency oscillation. The inlet is arranged on top so that the product can be poured onto the tray under gravity. The tray may be of metal, in which case it serves as the bottom of the chamber. It may also be of a dielectric material transmitting superhigh frequency oscillation.
The tray is provided with slanted openings for passage of air or gas forced into the chamber by a blower. The air drives granules over the surface of the stationary tray and out of the chamber and must be dry because apart from this function it also serves to remove moisture from granules.
The microwave chamber of the applicator may be mounted on shock absorbers. Vibrators may also be provided to vibrate the chamber.
In the latter case granules are moved by vibration, but vibrators can be used in combination with a blower.
A multimode resonance chamber can provide a uniform heating only when the product is mixed. However, no mixing means is included in the apparatus under review.
Vibration and air blowing can ensure a uniform motion of the product only when the latter is moved at a high speed. However, a high speed precludes a complete drying of the product. With the product in slow motion, the air flow is bound to rip up the layer of the granulated product, an effect similar to that taking place in the course of fluidization. As a result, the motion of the product is not uniform and the drying is inadequate.
There is known a method of using superhigh frequency oscillation for drying granulated polymer materials (cf. U.S. Pat. No. 3,771,234 of Sept. 9, 1969), such as butadienestyrene latex, polyvinyl chloride and polyisobutylene. According to this method, a hot air flow is used to bring granules to a heating zone. At first, the heating of the granules is carried out at a frequency of 915 megs during a time sufficient to reduce the moisture content to 5 percent, whereupon the heating is carried out at a frequency of 2,450 megs during a time sufficient to reduce the moisture content to 5,000 p.p.m., i.e. 0.5 percent. Upon drying the material, the air is removed with the aid of a cyclone-type device.
The drying time is 1 to 10 seconds at the first stage, and 1 to 3 seconds at the second stage.
The granulated material moves at a speed of a few tens of meters per second, which means that the drying temperature should be reached during a few seconds. This necessitates a considerable superhigh frequency power input, as well as high temperature air used to transport the granules. Destruction of the granules due to the intensified evaporation is avoided by providing for a positive temperature gradient .DELTA.t=i.multidot.n .degree. C. of the granules, where i is the mass and heat transfer vector. With a positive .DELTA.t, the temperature inside the granules is at least a few degrees lower then the temperature of the outer surface of the granules; under such conditions, it is only the outer surface of a granule that rapidly loses moisture.
An accelerated removal of moisture from the inside of the granules makes it necessary to equalize the inside and outer surface temperatures. It would be ideal to have a negative temperature gradient .DELTA.t in the granules, at which the internal temperature of a granule is higher than the temperature of its outer surface. However, the method under review does not make it possible to attain this goal. For example, according to this method, butadiene-styrene latex with an original moisture content of 20 percent is dried in two zones of a waveguide channel having a diameter of 3.5 inches and a total length of 150 feet. In the first zone, the heating is effected at a frequency of 915 megs (L-range) and the power input is 75 kw. In the second zone, the heating is carried out at a frequency of 2,450 megs (S-range) and the power input is 25 kw. The heating is further intensified by a flow of air, whereof the temperature is 250.degree. F. and the flow rate is 400 m.sup.3 per hour. The time of interaction between the granules and the superhigh frequency field is not greater than 5 seconds. The throughput is 1,000 pounds per hour.
PVC Granules are dried with a superhigh frequency power input of 60 kw; the air temperature is 145.degree. F. and the flow rate is about 200 m.sup.3 per hour. In the case of PVC, the time of interaction between the granules and the superhigh frequency field is 4 seconds, and the throughput is 500 pounds per hour.
The method under review is such that a granulated material is only dried until the granules can absorb a considerable amount of superhigh frequency oscillation energy and thermal energy of the air flow, i.e. until the permittivity .epsilon.' and the tangent of the dielectric loss angle tg .delta. are sufficiently great. The power input used for drying the above-mentioned materials is enough the remove the unbound moisture.
The complete drying, i.e. the removal of the capillary and absorption moisture, takes much more time than the preliminary drying, although the content of these types of moisture is normally not greater than 1 percent.
The method under review is carried out by using an apparatus for drying granulated dielectric materials, comprising a drying chamber which is a waveguide of a round section divided into two zones by an oscillation suppressor. Each zone includes several inputs to supply energy produced by superhigh frequency generators. Arranged at the inlet of the first zone are an air blower intended for pneumatic transportation of granules, and a granules charging means. Arranged at the outlet of the second zone of the waveguide is a cyclone separator intended to separate the air flow from the flow of granules. The separator is provided with an exhaust blower. The length of the waveguide's first zone is 20 to 100 feet; the heating in this zone is carried out at a frequency of 915 megs. The length of the second zone is 50 to 80 feet, and the frequency in this zone is 2,450 megs. At the inlet of the first zone, the moist granulated material is caught by the air flow and subjected to preliminary drying while passing through the first zone, complete drying taking place in the second zone. After a period of 3 to 5 seconds, granules are fed to the cyclone separator, wherefrom spent humid air is removed. At this point the material is ready for use.
The apparatus under review can only remove the unbound moisture from granules, i.e. it can only carry out preliminary drying. The time of interaction between the granules and the superhigh frequency field is variable within a period of a few seconds. It must be borne in mind in this connection that complete drying requires several minutes and in some cases even several tens of minutes. In the course of drying, the air used for the transportation of granules becomes humid, in which state it carries the granules all the way from the inlet to the outlet. Over a considerable portion of the first zone, the air temperature is higher than that of the granules, these temperatures being equalized in the second zone. As a result, there is a secondary interaction between the moisture and moist granules, which slows down the drying process and reduces the efficiency of the apparatus.